High-speed photographic emulsion and method of using same

ABSTRACT

A projection-speed silver bromide emulsion of washed type and containing by weight of the silver nitrate used in making the emulsion of an organic antifoggant in excess of 1.5 percent, a retarder consisting of a crystalline quaternary ammonium salt such as di-isobutyl phenoxy (or cresoxy) ethoxy ethyl dimethyl benzyl ammonium chloride monohydrate of approximately 0.1 percent (which also serves as a germicide), and a hardener ranging between 0.04 and 0.15 of the total weight of the gelatin in the emulsion.

United States Patent [72] inventor [54] HIGH-SPEED PHOTOGRAPHIC EMULSIONAND METHOD OF USING SAME 2 Claims, No Drawings [52] U.S. CL. 96/364 [51]Int. Cl G03c 5/00 [50] Field of Search 96/364, 94,

[56] References Cited UNITED STATES PATENTS 2,694,637 11/1954 Gray96/114.7 2,969,014 1/1961 Hanson 96/364 3,458,317 7/1969 Ditzer 96/94OTHER REFERENCES Photographic Chemistry," Pierre Glafkides, pp. 375,380, 381, 382, 383 (1957) Primary ExaminerNorman G. Torchin AssistantExaminer-Judson R. l-lightower Attorney-Charles E. Temko ABSTRACT: Aprojection-speed silver bromide emulsion of washed type and containingby weight of the silver nitrate used in making the emulsion of anorganic antifoggant in excess of 1.5 percent, a retarder consisting of acrystalline quaternary ammonium salt such as di-isobutyl phenoxy (orcresoxy) ethoxy ethyl dimethyl benzyl ammonium chloride monohydrate ofapproximately 0.1 percent (which also serves as a germicide), and ahardener ranging between 0.04 and 0.15 of the total weight of thegelatin in the emulsion.

stencil material coated with such emulsion directly in the copy camera.

In the making of silk screen stencils by photographic methods, the usualprocess employs contact speed emulsions of dichromate type,necessitating the making of a film positive in the same size as thescreen. The film positive is then placed over the screen and a brilliantlight source is used to directly harden theemulsion in the areas wherethe light penetrates the film,

In accordance with my invention, and the process using it, a coatedscreen may be placed directly in the process camera where the film wouldordinarily be placed in conventional methods. The screen then registersthe image directly from the original art work, following which thescreen is given a developer bath with a sponge and washed out in wannwater. The result is a screen that is a negative stencil of the originalartwork, and can be used for printing purposes. An alternative method ofsilk screen making utilizing a projection speed emulsion entails the useof a small film positive in an enlarger. The film positive is thereforeprojected onto the screen, enabling large silk screens to be made withsmall film positives. The significance of improved speeds in such anemulsion is that a totally different method of processing can beobtained, one that eliminates a great deal of time and effort. This isan obvious advantage in economy as well as time, since traditionalmethods dictate the use of a large film positive and a vacuum frame, inaddition to large lighting equipment to make contact process screens.

It is therefore among the principal objects of the present invention toprovide an improved high speed silver bromide emulsion, whereby thedirect exposure of the sensitized silk screen to art copy is possible.

Another object of the invention lies in the provision of an improvedmethod utilizing the above high-speed emulsion, with consequentsubstantial savings in time and expense.

These objects, as well as other incidental ends and advantages, willmore fully appear in the progress of the following disclosure, and bepointed out in the appended claims.

Briefly stated, the invention contemplates the making of a silverbromide emulsion in a gelatin suspension. Tl-le emulsion is preferablywashed, that is to say, it contains little or no potassium nitrate as aresult of rinsing away by water. The resultant pure silver bromideemulsion is of a type which is commonly used for black and white film,and also for coating fast enlargement papers. The bromide emulsion iscoated on the silk screen, and after exposure the screen is rinsed witha developing solution containing pyrogallol and metol, plus a base. Thepurpose of the development is to both reduce the latent image to puresilver, and also to tan the gelatin locally in the pure silver area.Pyrogallol is one of the developers which .has the capacity of tanningthe gelatin in the presence of pure silver. Other well-known developerssuch as hydroquinone and pyrocatechin may also be used, however. Theportions of the emulsion that have been tanned by the developer thusbecome insoluble, due to the tanning of the gelatin. When the screen isplaced in warm water, the gelatin which has not been tanned washes away,leaving the hardened gelatin. Thus, it may be observed that thedeveloper acts simultaneously to reduce the lightactivated silverbromide to metallic silver, and tan the gelatin locally in the presenceof reduced silver.

ln the above described process, the substitution of a highspeed silverbromide emulsion for the more traditional dichromate emulsion is notwithout substantial modification of conventional silver bromide type ofemulsions. Traditionally, the contact speed emulsions have been employedin the silk screen process for the reason that such emulsions possesssubstantially superior contrast characteristics which facilitates thetanning action and subsequent removal of unwanted gelatin. So that thecontrast qualities of the silver bromide emulsion employed are adequatefor the present purpose, after washing to increase speed, substantialquantities of restrainer and organic antifoggant are added to theemulsion. Certain quaternary ammonium salts make an excellentrestrainer, and is required to exceed approximately 0.1 percent byweight of the silver nitrate used in making the emulsion. The amount oforganic antifoggant, preferably benzotriazol must exceed 1.5 percent thetotal weight of silver nitrate used in making the emulsion, thepreferred range being from 1.5 percent to 2.0 percent. For differentrestrainers and/or antifoggants, different percentages would apply. ineach case, however, the quantities required are substantially greaterthan those of conventional high-speed photographic emulsions.

A second distinctive characteristic of the emulsion is the presence of ahardener in the finished emulsion. The hardener must not be present insuch great quantity as to insolubilize the gelatin, as is the case withhardeners that are added before emulsions are coated on paper or film,but sufficient to substantially aid the tanning process encountered ondeveloping after exposure. In the case of potassium chrome alum, whenused as the sole hardener in the emulsion, the proportion should not beless than 0.4 percent and should not exceed 0. l 5 percent of the totalweight of gelatin in the emulsion. The reason for this critical factoris that the tanning developers which are used, e.g., pyrogallol withmetol, are not sufficiently powerful to provide sufficient tanning of anemulsion without some hardener. However, a hardener in quantities abovethat indicated would hamper clean and easy washout of the screen andwould, in a sufficient quantity, cause the emulsion eventually to bedifficult to liquify at the temperatures which would be harmless to theemulsion by considerably raising the melting point of the gelatin beyonda practical range. Further, the gelatin content of the emulsion must begreater than that used for paper or film, in the order of at least 200grams of gelatin per liter of the emulsion. lf substantially lessgelatin is used, the emulsion will be difficult to coat as it will flowthrough the screen, and if substantially greater quantities are used,the tanning action will be impaired. In summary, therefore, it ispossible to attribute certain distinctive characteristics to theemulsion. It is a washed emulsion, predominately bromide, with anunusually large amount of restrainer and/or antifoggant added forcontrast, and long shelf life, a critical amount of hardener, and arelatively high-gelatin content. A side benefit of the large amount ofrestrainer plus antifoggant is that the emulsion demonstrates unusuallygood storage properties, and can generally be kept for as long as 3months at normal room temperatures. This is due to the fog restrainingactivity of these types of compounds. The emulsions may, if desired,contain other additives, as do most photographic emulsions, suchasplasticizers and surface active agents to achieve greater ease ofhandling.

My process also lends itself to the production of reversed reliefs. Asis known in the art, not only can gelatin be tanned locally in thepresence of silver, but it may also be softened locally in the presenceof silver. This is customarily achieved in the art through the use ofhydrogen peroxide. Positive art work may thus be used to produce apositive screen. This aspect of the invention may be applied to screenmaking on a practical basis by using a film negative to project line orhalftone art work onto a screen to achieve a negative screen. Let usassume that a photographic amateur using ordinary film is interested inmaking silk screens. He may then photograph original art work (positive)using a conventional panchromatic film. The negative may then be placedin an enlarger, and a screen coated with the disclosed emulsion placedunder the enlarger prior to making exposure. Using the reverse reliefprocess above described, he may then produce a negative screencorresponding to the reverse of the original art work. This negativescreen is then used for printing purposes to produce a final positiveprint when inked. Thus, through reversal processing coupled with the useof this emulsion, a vastly wider range of silk screen processes may becreated. Note, that without reversal processing, the amateur would haveto obtain a film positive of the original art work, assuming that hecommenced with positive artwork, before placing it in the enlarger inorder to obtain a negative screen. Thus, reversal processing coupledwith the use of an enlargement speed emulsion are both necessary toprovide silk screen benefits to a much wider audience than is availablein the case of contact printing methods. Reversal processing is achievedthrough the conventional use of hydrogen peroxide (plus a halogenizingagent, in this case cupric chloride in an acid solution) suggestsanother important application area for my emulsion, which is the makingof photoengraving plates directly with a camera, and withoutintermediate steps connected with film making. The photoengraver simplycoats a metal plate with emulsion, and places it in the process camerawhere the film would ordinarily be located. He then photographs theoriginal art work (positive) onto the sensitized plate, using thereversal processing technique to make a positive photoresist plate. Theemulsion areas on the plate after processing will correspond to theunexposed portions of the emulsion. The coated plate may then behardened either chemically or through the use of moderate heat in orderto withstand the etchant solution. The use of pure gelatin as thecolloidal agent will not, of course, provide as great a degree ofresistance to etching as will other so-called enamels that are used asconventional engraving resists. However, gelatin does offer a degree ofresistance satisfactory for many needs, and if necessary, limitedamounts of other colloids, such as fish glue, may be added to theemulsion in order to increase adhesion to the plate and resistance tothe etching solution. Correspondingly, a greater amount of hardener mustbe incorporated into the emulsion used for plate making purposes than isthe case in the making of silk screen emulsions, in order to overcomethe tendency of the emulsion to lack adhesion to a smooth metal surface.However, the proportion of hardener assuming the use of chrome alumshould still not be so great as to prevent liquification of the emulsionat a temperature of 120 F. or less, which is the maximum for practicalpurposes of shelf life.

The techniques described here and above for silk screen orphotoengraving use may also be applied with suitable modificationthrough a wide range of graphic applications. Many printing processessuch as collotype, offset, intaglio, etc. require the use of aphotosensitive emulsion. The purpose may be to provide a relief surface,or to form a surface that attracts or repels greasy or water-base inks.The latter described procedure is the basis for the familiar photooffset printing process. In all of these cases, the basic emulsiondescribed herein is an appropriate agent.

The following examples are illustrative, and are given as specificexamples of exact methods of carrying out the invention. it will beunderstood that modifications within the scope of the invention arepossible by those skilled in the art to which the invention pertains.

EXAMPLE I g l A silver bromide emulsion is made by conventional methodsusing silver nitrate and potassium bromate, washing the same to removesubstantially all traces of potassium nitrate. To this emulsion, anorganic antifoggant benzotriazol is added, ranging from 1.5 percent to2.0 percent by weight of the silver nitrate ordinarily used, followed byapproximately 0.1 percent of a retarder and organic antifoggant,di-isobutyl phenoxy (or cresoxy) ethoxy ethyl dimethyl benzly ammoniumchloride monohydrate, similarly computed, and 0.04 percent to 0.15

usin spra ing, dipping, or doctor blade techniques, all of whic are nownm the art, following which the emulsion is permitted to cool and harden.

In the above described proportions of emulsion and gelatin, the preparedsuspension will have an ASA rating approximately the same as fastprocess film, permitting it to be directly exposed in a copy camera withrelatively short exposure time.

The developing and tanning process is best performed with a spongedipped in the developing solution, and following the reduction tometallic silver and subsequent tanning of the exposed portions of thegelatin, the remaining soft gelatin is gently flushed away with warmwater. The developing solution is a conventional metol-pyrogallol typewell-known in the art, or in the alternative, a metol-hydroquinonedeveloper may also be employed. As the hardener is incorporated directlyin the emulsion, no separate hardening and bleaching step is necessary,and after drying, the stencil is ready for immediate use.

EXAMPLE 2 A silver bromide emulsion is prepared as in Example 1, andcoated upon the stencil and exposed as in that example. After developingthe exposed emulsion with conventional (nontanning) paper developer, thestencil is subjected to the action of a mild acid solution of hydrogenperoxide and cupric chloride, which results in the softening of theexposed areas of the gelatin relative to the unexposed areas. In thiscase, the potassium chrome alum present in the emulsion willsufficiently harden the unexposed areas of the exposed image to permit areverse relief.

I wish it to be understood that I do not consider the invention limitedto the recise details of structure shown and set forth in thisspecification, for obvious modifications will occur to those skilled inthe art to which the invention pertains.

I claim:

1. The method of preparing a silk screen stencil photographically,including the steps of: preparing a high-speed silver bromide emulsion,washing said emulsion to substantially remove any potassium nitratepresent, mixing said emulsion with a relatively hard gelatin to form asuspension in the ratio of at least 200 grams of gelatin per liter ofemulsion, adding 0.04 percent to 0.15 percent of potassium chrome alum,approximately 1.5 percent benzotriazol, and approximately 0.1 percent ofa quaternary ammonium salt, each by weight of the amount of silvernitrate used; coating a porous stencil screen with said suspension toblock the mesh thereof, exposing said screen to a projected light image,subjecting the exposed screen to the action of a tanning developerselected from the group consisting of pyrogallol, hydroquinone andpyrocatechin, wherein exposed portions of the emulsion become insoluble,and subsequently subjecting the developed screen stencil to the actionof warm water whereby untanned gelatin is washed away.

2. The method in accordance with claim 1, including the step ofsubjecting the exposed and developed image to the action of agelatin-softening agent, including hydrogen peroxide.

2. The method in accordance with claim 1, including the step ofsubjecting the exposed and developed image to the action of agelatin-softening agent, including hydrogen peroxide.